It has been known for many years that blending fibres into polymers can significantly improve the mechanical properties of the polymers in question. Long fibres made of materials such as metal, glass or asbestos (GB 1179569 A) have been used to this effect. Boron, silicon carbide and even carbon fibres have been developed for this purpose. The initially developed carbon fibres had diameters of several tens of microns and lengths in the order of millimeters. They were quite light and despite this had impressive mechanical properties, displaying Young's moduli in the range of 230 to 725 GPa and strengths in the range of 1.5 to 4.8 GPa. Carbon fibres, also known as carbon nanofibres, having higher aspect ratios, have also been prepared having even smaller diameters of about 100 nm and lengths up to 100 microns, Young's moduli in the range of 100 to 1000 GPa and strengths in the range of 2.5 to 3.5 GPa.
However, the most recent development, resulting from the discovery of Buckminsterfullerene (C60), is the carbon nanotube having unprecedented physical and chemical properties. A single wall carbon nanotube (SWNT) is a one-atom thick sheet of graphite (called graphene) rolled up into a seamless hollow cylinder which can have a diameter of the order of 1 nm and lengths of up to several millimeters. The aspect ratio can thus potentially reach values of several millions. Multi-walled carbon nanotubes (MWNT) have also been developed, which are concentric arrays of single-walled carbon nanotubes (also known as the Russian Doll model).
With Young moduli of up to 5 TPa and mechanical strengths even greater than 70 GPa, carbon nanotubes have great potential to replace conventional carbon fibres as polymer reinforcements.
Carbon nanotubes are also extremely light and have unique thermal and electronic properties. Depending on how the graphene sheet is rolled i.e. the relationship between the axial direction and the unit vectors describing the hexagonal lattice, and depending on the diameter, on the number of walls and on the helicity, the nanotube can be designed to be conducting or semi-conducting. Carbon nanotubes of high purity are extremely conductive. In theory, pristine carbon nanotubes should be able to have an electrical current density of more than 1,000 times greater than metals such as silver and copper. This is because no scattering of charge occurs as it travels through the tube, resulting in what is known as ballistic transport of the charge. Nanotubes may thus be added to an electrically insulating polymer to produce conductive plastics with exceedingly low percolation thresholds as described in WO 97/15934.
As for thermal properties, carbon nanotubes are also very conductive for phonons. Previous calculations predict that at room temperature, thermal conductivity of up to 6000 W/m K can be achieved with pure nanotubes, which is roughly twice as much as pure diamond. Nanotubes dispersed within a polymer matrix can thus provide thermally conductive resin compositions.
Carbon nanotubes have also been cited as having flame retardant properties. Nanotubes dispersed within a polymer matrix can thus provide materials with fire proof properties.
Due to all of these properties, carbon nanotubes have been envisaged for use in many applications in recent years (see P. Calvert “Potential application of nanotubes” in Carbon Nanotubes, Editor T. W. Ebbeson, 297, CRC, Boca Raton, Fla. 1997; T. W. Ebbeson, “Carbon Nanotubes”, Annu. Rev. Mater. Sci., 24, 235, 1994; Robert F. Service, “Super strong nanotubes show they are smart too”, Science, 281, 940, 1998; and B. I. Yakobson and R. E. Smalley, “Une technologie pour le troisième millénaire: les nanotubes”, La Recherche, 307, 50, 1998). However, currently the most promising line of research involves the mechanical enhancement of polymers by using carbon nanotubes as reinforcing fillers.
Overall, it can be considered that there are four main requirements for the carbon nanotubes to effectively reinforce the polymer and to increase its conductivity and flame retardation properties: good dispersion of the nanotubes in the polymer matrix, large aspect ratio of the nanotubes, efficient transfer of interfacial stress and alignment (Coleman et al., Carbon, 44, 2006, pp 1624-1652). Good dispersion can be considered to be the most important factor. The blends of carbon nanotubes and polymer must be homogeneous i.e. the nanotubes must be uniformly dispersed with the effect that each nanotube is individually coated with the polymer so that efficient load transfer to the nanotube network can be achieved. Lack of homogeneity introduces stress concentration centres i.e. weak points where there is, for instance, a relatively low concentration of nanotubes and a high concentration of polymer. Non-homogeneous nanotube-polymer composites therefore result in only slight improvements in mechanical strength and little or no improvement of electrical conductivity.
One of the current areas of research is the carbon nanotube reinforcement of thermoplastic polymers, in particular of commodity plastics such as polyolefins. However, as of yet melt processed blends of polyolefins and carbon nanotubes have not produced the desired results. Dispersion of the nanotubes in the polyolefin matrix using conventional techniques is poor due to the presence of Van der Waals interactions that favour the formation of carbon nanotube agglomerates. Other methods for blending, such as solution processing, surfactant-assisted processing, solution-evaporation methods with high-energy sonication and the like, which break up, to an extent, the agglomerates, have provided slightly better results. However, these are time-, energy- and money-consuming processes. There is thus a need to improve the results from melt processing, as it is the most preferred industrial method for blending due to its speed, simplicity and compatibility with standard industrial equipment. As a result of poor dispersion from melt processing, the mechanical strengths and Young's moduli of the nanotube-polyolefin composites are not increased to the extent expected and in certain cases are even decreased. Electrical conductivity of the resulting non-homogeneous composite is also lower than expected. Those procedures reported as providing homogeneous nanotube-polyolefin composites are often misleading, since in these aggressive blending methods, the nanotubes are forced to break, thereby lowering the aspect ratio and limiting the potential increase in stiffness, strength and conductivity of the composite. Aggressive blending can also result in the damaging of the surface of the carbon nanotubes, which also lowers stability and conductivity of the composite.
Tang et al. Carbon, 41, 2003, 2779-2785 report a MWNT/HDPE composite, which was compounded using a twin-screw extruder after preliminary melt-mixing. From the figures in the relevant article it is seen that while there are some individual nanotubes scattered in the matrix, most of them are clumped together forming large aggregates. An attempt was made to co-feed the MWNTs directly into the extruder, but this technique had to be abandoned as the MWNTs had a tendency to stick to the hopper walls. Overall mechanical properties improved, but not to the extent that would be expected of carbon nanotubes. This is probably due to the problems mentioned above regarding reduced aspect ratios and damaged surfaces resulting from aggressive blending methods.
Lopez Manchado et al. Carbon 43, 2005, 1499-1505 reported a melt compounded polypropylene and carbon nanotube composite. Low concentrations of nanotubes less than 1 wt % showed some improvement in mechanical properties. However, large aggregates of nanotubes were already observed when only 1 wt % of carbon nanotubes was present, at which point stiffness and strength of the composite decreased.
EP 1 181 331 also discloses composites of carbon nanotubes and polyolefins, whereby the mixture is stretched both in the molten state and in the solidified state to increase alignment of the carbon nanotubes and thereby induce higher mechanical strengths in the composite therefrom. However, while stretching blends to orientate the carbon nanotubes is a method suitable for fibre applications and to maximize the mechanical properties of fibres, this may not be the case for other applications. Aligned composites have an-isotropic mechanical properties that may need to be prevented in bulk samples.
EP 1 349 179 discloses partly purified carbon nanotubes i.e. carbon nanotubes that have not been partially oxidized during a purification step. It is shown that these nanotubes have a better dispersion in apolar polymers such as polyolefins. Oxidised nanotubes have altered polarities and hence a reduced affinity to apolar polymers, such as polypropylene and polyethylene. However, in the figures one can still see the agglomerates of nanotubes in the nanoscale. There is thus a need to induce dispersion of nanotubes in the nanoscale in polyolefins.
Another method of enhancing miscibility of carbon nanotubes with polyolefins has been by functionalizing the carbon nanotubes and/or the polyolefins. Functionalisation is described in J. Chen et al., Science, 282, 95-98, 1998; Y. Chen et al., J. Mater. Res., 13, 2423-2431, 1998; M. A. Hamon et al., Adv. Mater., 11, 834-840, 1999; A. Hiroki et al., J. Phys. Chem. B, 103, 8116-8121, 1999. The functionalisation can be carried out, for instance, by reaction with an alkylamine. It results in a better separation of the nanotubes in the polypropylene matrix thereby favouring the dispersion in the polymer matrix. If the functionalisation is carried out in both the nanotubes and the polymer matrix it promotes their covalent bonding, thereby improving the electrical and mechanical properties of the filled compound. However, functionalisation requires a further reaction step, possibly even a further second step, if the polymer is to be functionalised too. This makes the overall process complicated and costly and in general unsuitable for large-scale industrial production. Furthermore, functionalisation can change the physical properties of the nanotubes, reducing their mechanical strength and electrical conductivity.
It is hence an object of the invention to produce carbon nanotube-poly(hydroxy carboxylic acid) composites that are highly homogeneous.
It is therefore also an aim of the invention to enhance the dispersion of nanotubes in polyolefins in the nanoscale.
Furthermore, it is an aim of the invention to obtain a homogeneous nanotube-poly(hydroxy carboxylic acid) composite by melt processing.
Additionally, it is an aim of the invention to blend the carbon nanotubes with the polyolefin without requiring a functionalisation or modification step respectively.
It is also an object of the invention to provide a resin with better mechanical properties than polyolefins.
It is further an object of the invention to render electrically insulating compositions comprising polyolefins more electrically conductive using carbon nanotubes.
Yet another aim of the invention is to increase the thermal conductivity of polyolefins with the effective dispersion of carbon nanotubes therein.